Fix Gn Untracked headers in webrtc/common_audio

Fixed following headers in this CL
===================================
src/webrtc/common_audio/vad/mock/mock_vad.h
src/webrtc/common_audio/mocks/mock_smoothing_filter.h
src/webrtc/common_audio/signal_processing/include/spl_inl_armv7.h

BUG=webrtc:7648

Review-Url: https://codereview.webrtc.org/3013063002
Cr-Original-Commit-Position: refs/heads/master@{#19824}
Review-Url: https://codereview.webrtc.org/3013673002
Cr-Commit-Position: refs/heads/master@{#19852}
diff --git a/common_audio/BUILD.gn b/common_audio/BUILD.gn
index 8cfd642..afb5e92 100644
--- a/common_audio/BUILD.gn
+++ b/common_audio/BUILD.gn
@@ -104,6 +104,18 @@
   }
 }
 
+rtc_source_set("mock_common_audio") {
+  testonly = true
+  sources = [
+    "mocks/mock_smoothing_filter.h",
+    "vad/mock/mock_vad.h",
+  ]
+  deps = [
+    ":common_audio",
+    "../test:test_support",
+  ]
+}
+
 rtc_source_set("common_audio_c_arm_asm") {
   visibility = [ ":*" ]  # Only targets in this file can depend on this.
   sources = []
@@ -144,6 +156,7 @@
     "signal_processing/include/real_fft.h",
     "signal_processing/include/signal_processing_library.h",
     "signal_processing/include/spl_inl.h",
+    "signal_processing/include/spl_inl_armv7.h",
     "signal_processing/levinson_durbin.c",
     "signal_processing/lpc_to_refl_coef.c",
     "signal_processing/min_max_operations.c",
diff --git a/modules/audio_coding/BUILD.gn b/modules/audio_coding/BUILD.gn
index ffa88af..2e06649 100644
--- a/modules/audio_coding/BUILD.gn
+++ b/modules/audio_coding/BUILD.gn
@@ -2202,6 +2202,7 @@
       "../../api/audio_codecs:builtin_audio_decoder_factory",
       "../../api/audio_codecs:builtin_audio_encoder_factory",
       "../../common_audio",
+      "../../common_audio:mock_common_audio",
       "../../rtc_base:protobuf_utils",
       "../../rtc_base:rtc_base",
       "../../rtc_base:rtc_base_approved",